The fundamental principle of generating symmetric cryptographic keys based on physical properties of the communication channel is described in literature. The use in principle of a helper node (trusted device, helper device) is also available in cryptography and was proposed in literature for channel-based key generation, for example, to incorporate the physical properties of further channels into the key generation or to increase the range of the system.
German Patent Application No. DE 10 2012 215 326 A1 describes a method for generating a cryptographic key in a network including a first network element, a second network element, and a network node, the first network element being able to communicate via a first transmission channel and the second network element being able to communicate via a second transmission channel with the network node. The method includes, on the part of the first network element, a step of determining a first item of channel information with respect to the first transmission channel based on a first pilot signal, which is emitted by the network node, and a step of ascertaining the symmetric cryptographic key using the first item of channel information and an item of information about a combined item of channel information, the combined item of channel information representing a combination, which is determined on the part of the network node based on a second pilot signal transmitted from the first network element to the network node and a third pilot signal transmitted from the second network element to the network node, of transmission characteristics of the first and the second transmission channels.